When you enter the dates under "Invoice Date" and "GL Post Date" it can not be Before the date of receiving.  If you received on 5/5/08 and you enter say 5/2/08 as the "Invoice Date" or "GL Post Date" it will give you that error.  You would need to put in 5/5/08 instead.  When you "view" the recievers look at the date of entry and that is the date you need to use (or a later date).

Dennis
 
When your display shows the  transaction occured on 4/08/08 but you are entering in the invoice date of 03/27/08 and post date of 05/06/08.  Then you can not use the 03/27/08 as the invoice date.  You have to use 4/08/08 as the earliest invoice date. You can not use any date that happened before any type of transaction actually took place.  I understand you actually initally received it on 3/31/08 but Classic is not recognizing it until you release from QC Inspection as it as at that point it is released to inventory/work order.  
 
In a way it is the same scenario the date of release from QC Inspection to Inventory/Work Order (for the service po) it the date of actual recognition not the date it was received in to QC.
 
Hope that helps.